How do I simplify the polygons of a 3D scanned model?
One way to simplify geometry would be to use the Decimate Modifier.

Add the Decimate modifier set to Collapse, and turn down the ratio.
Click Apply, if you want to edit your geometry manually after.



If you want to see the polygons like in my gif, enable Wireframe in the Overlay menu.



Then just manually select and join faces into one.
Vertex > New Edges/Faces from Vertices
Shortcut: F

  

There are also other ways to simplify geometry, I didn't want to overwhelm you.